variable: disable tidb_read_staleness for SET_VAR hint (#48477)

[ti-chi-bot]

This is an automated cherry-pick of #48477

What problem does this PR solve?

Issue Number: close #48478

Problem Summary:

What is changed and how it works?

This variable takes effect at the planner's preprocess phase. But we build SET_VAR hint just before we parse the AST node.

We disable it first to make the behavior consistent.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hawkingrei, winoros

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-11-09 13:39:04.751011587 +0000 UTC m=+3737942.338121743: ☑️ agreed by hawkingrei.
• 2023-11-09 13:40:43.114323414 +0000 UTC m=+3738040.701433558: ☑️ agreed by winoros.